The invention pertains to the treatment of wastewater to remove pollutants. Specifically, in one of its aspects, the invention provides an improved method for removing organic matter from domestic wastewater, consolidating and digesting the removed solids. More specifically, the improved method employs a treatment tank having an upper sedimentation zone for removing particulate organic matter from wastewater that utilizes a substantially reduced wastewater detention time and a lower subjacent solids accumulation zone wherein the settled matter is digested. The reduced wastewater detention time in the upper sedimentation zone of the new method minimizes the amount of dissolved and/or entrained gasses, particularly hydrogen sulphide in the wastewater effluent from the treatment tank.
A downstream secondary biological treatment unit to remove and oxidize organic matter alone or both organic and nitrogenous pollutants can also be provided in combination with the improved treatment tank. Further, the solids captured and generated by the downstream biological treatment can be returned to the influent of the improved treatment tank for separation and consolidation.
The combined functions of settling particulate organic matter from wastewater and biochemically converting the accumulated matter to sludge is well known in the wastewater treatment art. Apparatus of this type are generally known in the art as septic tanks and are shown in Imhoff U.S. Pat. Nos. 978,889, 1,642,206; 1,692,446 and 1,925,679; Howe et al. U.S. Pat. No. 2,272,004 and Allen U.S. Pat. No. 3,251,471 among others.
Septic tanks function to remove settleable organic solids, grit, screenable materials and grease from wastewater as well as to provide for the digestion of settled organic matter by anaerobic bacterial decomposition. To separate the solids removal function from the digestion, a flow-through chamber is constructed at the tank surface in such manner that the solids removed therein are conveyed to the lower zone. Apparatus of this type as described in the aforementioned patents typically comprise horizontally disposed chambers in vertical juxtapostion and in hydraulic communication with each other, the upper chamber functioning as a settling or clarifying station through which wastewater is passed directly from the influent means of the combined unit to the effluent means thereof. The lower chamber functions to anaerobically decompose solids removed by gravity from the wastewater in the course of its passage through the upper chamber. Septic tanks of this type are commonly referred to in the art as Imhoff tanks.
In the operation of a typical septic tank the influent wastewater is detained in the apparatus for about 24 hours during which time grease, grit and particulate organic matter are removed by being settled to the bottom. The accumulated solids at the bottom of the tank are anaerobically decomposed with the generation of gasses such as methane and carbon dioxide. As much as one half of the accumulated organic solids are converted to gaseous products by digestion. During warmer weather, when the wastewater temperature rises above 60.degree.F., the generation of hydrogen sulphide gas becomes evident. As the temperature increases, the volume of hydrogen sulphide gas produced by the anaerobic decomposition increases. Since hydrogen sulphide is fairly soluble in water, a substantial portion of this gas that is generated is contained in the effluent wastewater. Typically, the effluent from a septic tank is percolated into the ground and its hydrogen sulphide content is of little, if any, consequence. The digested sludge is periodically removed from the unit and disposed of by various means well known in the art.
Although septic tanks are effective in removing and decomposing organic solids present in wastewater such as normal domestic wastewater, the effluent from units of this type contain significant concentrations of soluble carbonaceous matter, usually expressed BOD (Biological Oxygen Demand) and oxidizable nitrogenous matter, primarily ammonia and albuminoid nitrogen. Septic tank effluent that is percolated into the ground can find its way to potable underground water supplies as well as lakes, rivers and streams, depending on the characteristics of the soil and the design of the septic system. Governmental regulations, particularly where potable water supplies are involved, make the secondary treatment of septic tank effluent to remove soluble carbonaceous and nitrogenous matter mandatory where the percolation capacity of the soil is not satisfactory or becomes exhausted.
A primary objective of the invention is to provide a method of wastewater treatment employing a modified septic tank to remove and digest solid organic pollutants as well as to remove sand, grease, grit, etc. in combination with a secondary treatment unit to biochemically remove or oxidize soluble carbonaceous matter alone or in addition to the nitrogenous matter present in the septic tank effluent. Secondary treatment facilities known in the art include the activated sludge process wherein suspended biological forms are utilized to oxidize carbonaceous pollutants; the trickling filtration process wherein fixed biological slimes are grown on a stationary media and the rotating biological contactor process wherein biological slimes are grown on a stationary media and the rotating biological contactor process wherein biological slimes are grown on rotating surfaces that are alternately exposed to the atmosphere and immersed in the wastewater.
A preferred biological process for treating wastewater to oxidize carbonaceous matter utilizes rotating biological contactors in the form of a plurality of closely spaced, partially submerged bodies, to grow fixed biological slimes. The rotating biological contactors can be in the form of thin discs, drums, cylinders, brushes, etc. A preferred contactor has a discontinuous honeycomb-type structure and is disclosed in U.S. Pat. application Ser. No. 252,038 filed May 10, 1972.
The partially submerged bodies are forcibly rotated to alternately expose the organisms on the contactor surfaces to the atmosphere for the absorption of oxygen and the wastewater for nutrients at a peripheral velocity suitable for the growth and maintenance of aerobic biological organisms. Such rotating biological contactors are typically arranged to subject the wastewater to sequential treatment by 3 to 6 stages arranged in series. Secondary treatment systems of this type are capable of oxidizing solubilized carbonaceous and nitrogenous pollutants and of removing 95% BOD5 from the wastewaters depending on the specific operating conditions.
Representative wastewater treatment systems utilizing a multistage configuration of partially submerged rotating biological contactors for the sequential treatment of wastewater are shown and described in Torpey U.S. Pat. No. 3,575,849; EL Naggar U.S. Pat. No. 3,335,08l; Simpson U.S. Pat. No. 3,466,241; and Hartmann et al. U.S. Pat. No. 3,389,798 among others. Rotating biological contactors can also be operated to simultaneously remove or oxidize carbonaceous and nitrogenous pollutants as disclosed in detail in the applicant's U.S. Pat. No. 3,817,857.
A further objective of the invention is to provide a method of wastewater treatment including the removal of solids and the anaerobic decomposition of settled organic solids wherein the wastewater effluent is substantially free from dissolved or entrained hydrogen sulphide. Although the use of a biological treatment unit employing rotating biological contactors for the secondary treatment of septic tank effluent is advantageous, a serious efficiency reducing problem develops when the influent wastewater to the biological treatment unit i.e. septic tank effluent, contains a significant amount of hydrogen sulphide, which would be the case during normal operation of a septic tank at a wastewater temperature above about 60.degree.F.
The presence of hydrogen sulphide in the influent wastewater to the biological treatment unit employing surface area for the growth and maintenance of biological slimes creates an environment wherein sulfur-oxidizing filamentous organisms known as beggiatoa dominate and capture a significant portion of surface area thereby leaving a lesser amount of surface available for the growth and maintenance of organisms that oxidize carbonaceous and nitrogenous pollutants. Hydrogen sulphide presence in the biological treatment unit interferes with the oxidation of ammonia and carbonaceous matter by occupying surface area resulting in decreased effectiveness of the secondary treatment. This problem is particularly serious when the surface area is in the form of rotating contactors.